How to get reflections from 3D to show up on real images
Use emission shaders as a way to create a mask for the reflections.
(for more information see this answer)



Use two layers for the 3D layout:

Layer 1, containing the scene with all your objects, lights and reflective surfaces

Layer 2, containing the reflective surfaces and a copy of the objects to be reflected, but this copies should have an emission shader only.

On the render layers section create two Layers:

One render layer (I'll call it color) that holds the scene's Layer 1, excludes layer 2 and uses the combined pass.

A second render layer (I'll call it emitter reflection mask) that holds the scene's layer 2, excludes layer 1 and has Glossy Direct enabled, as well as Emission.

 

On the compositor, Use a color mix node to add the Emit and Glossy direct form the Emitter reflections mask layer. Use the result as alpha channel to composite the color pass over the backgorund image.





To make the reflection less perfect (which I think is what @troy_s suggested) you can take the Glossy direct information,  multiply it with some gray color in a color mix node, and add that to the composited image(You might even want to add some blur or other textures along the node tree too)...



Another example

(using the same scene as in this answer). Simple geometry.

On layer 1 just a plane to catch the reflections and the reflected object, lit by emitting planes with the background image.

On layer 2 the same scene, but with a copy of the object using an emission shader and no other lights.



Set the render layers as described above, that will allow us able to create mattes to separate the original object from the reflected one, and use those mattes to make the final combination of the CG element and the Image background.



The easy part is using the Emssions of the second layer to isolate the forground element.



If we were to compose that over the background using alpha over we'd get some refection... but it would be semi-transparent, making it unrealistic. In real life the reflection of the object would occlude the background...



To get rid of the reflections of the lights on the ground we need a piece that is clean of those reflections but has the reflection of our 3D object.
(I used a portion of right side of the image that does not have the reflection of the lights, that's why the rotate and transform node are for, but in other cases you might need to use photoshop or other image editor to crate a "clean" plate).

Then, combine the background image with the reflection.



Now mix it all together using the glossy direct information as factor for the mix:





To make the reflection on the less present you can change the factor level on the mix with the background plate: